"Stressed-out" persons secrete excessive amounts of steroids, one of which is cortisol. Cortisol suppresses the immune response. This response explains, in part, why persons who are stressed out experience a higher rate of upper respiratory infections. Some scientists have suggested that high levels of stress might encourage the development of cancer.

The shark skeleton is composed entirely of cartilage. Because sharks do not develop cancer, it is assumed that the shark cartilage contains a tumor-suppressing agent. In recent years, shark cartilage has been ground up and sold as a tumor suppressant.